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A number of recent studies have shown that young healthy women may be susceptible to pulmonary system limitations during exercise including expiratory flow limitation (EFL). Several sex based differences in the anatomy and function of the pulmonary system have been reported as possible mechanisms. For example, women consistently have smaller lung volumes, smaller diameter airways, and a decreased diffusion surface area relative to height matched men. These anatomical differences may also have implications in terms of breathing mechanics, particularly at maximal exercise. However, there have been no studies that have systematically compared the mechanics of breathing in men and women. Accordingly, the purpose of the present study was to provide a comprehensive assessment of breathing mechanics including the measurement of EFL, end expiratory lung volume (EELV), end inspiratory lung volume (EILV), and the work of breathing (Wb) in endurance trained men and women. It was hypothesized that women would develop EFL more frequently than men and that women would have greater relative increases in EELV and EILV at maximal exercise. It was also hypothesized that women would have a higher Wb across a range of ventilations. EFL was assessed by applying a negative expiratory pressure (NEP) at the mouth and comparing the resultant flow volume curve with that of the preceding control breath. If the NEP increased expiratory flow rate, the subject was considered non flow limited. Conversely, if application of the NEP did not illicit an increase in expiratory flow, the subject was considered flow limited. Operational lung volumes (i.e., EELV and EILV) were determined at various stages of exercise by having subjects perform inspiratory capacity manoeuvres. Flow, volume, oesophageal and airway opening pressure were continuously monitored throughout exercise. Trans-pulmonary pressure (Ptp) was taken as the difference between oesophageal and airway opening pressure which was then plotted against volume. The integral of the Ptp-volume loop was multiplied by breathing frequency to determine the Wb. A total of 18 endurance trained male (n=8) and female (n=10) athletes volunteered to participate in this study. Males had a higher absolute (mean ± SD; 5.30 ± 0.7 vs. 3.8 ± 0.4 L min⁻¹) and relative (69.5 ± 7.8 vs. 59.8 ± 4.8 mL-kg-¹.min-¹) VOE₂MAX and a higher maximal minute ventilation (161 ± 25 vs. 120 ± 18 L.min-¹) compared to females (P<0.01). Due to an abnormal response to the NEP in one male subject, EFL data was obtained in 7 of the 8 males. EFL occurred in 9 females (90%) and 4 males (57%) during the final stage of exercise. However, 8 (6F, 2M) of these subjects were later able to overcome EFL during the final stage of exercise through an alteration in breathing pattern. Females had a higher relative EELV (42±8 vs. 35±5 %FVC) and EILV (88±5v s. 82±7 %FVC) compared to males at maximal exercise (P<0.05). Women also had a higher Wb compared to men across a range of ventilations. On average, women had a Wb that was twice that of men at ventilatioℓ̨̨ns above 90 L.rninw’. This data suggests that EFL may be more common in females and that they experience greater relative increases in EELV and EILV at maximal exercise compared to males. The higher Wb in women is likely attributed to their smaller lung volumes and smaller diameter airways. Collectively, these findings suggest that women utilize the majority of their ventilatory reserve compared to men but the associated cost may have physiological and performance based implications.

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